Add Pending changelists view (option B)

The Changes tab now shows committed-but-not-submitted changelists as cards
above the working set: expandable to their files (click to preview), each
with per-CL Submit, plus Edit description / Uncommit (reopen -c default) /
Discard in a ⋯ menu, and a Submit-all button. Backend adds p4_reopen_default
and p4_set_desc; a small text-prompt modal edits descriptions. Preview panel
now shows either the working selection or a clicked pending-CL file.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/p4.ts

@@ -64,6 +64,8 @@ export const p4 = {
   commit: (description: string, files: string[]) =>
     invoke<string>("p4_commit", { description, files }),
   submitChange: (change: string) => invoke<string>("p4_submit_change", { change }),
+  reopenDefault: (files: string[]) => invoke<string>("p4_reopen_default", { files }),
+  setDesc: (change: string, description: string) => invoke<string>("p4_set_desc", { change, description }),
   add: (files: string[]) => invoke<OpenedFile[]>("p4_add", { files }),
   edit: (files: string[]) => invoke<OpenedFile[]>("p4_edit", { files }),
   del: (files: string[]) => invoke<OpenedFile[]>("p4_delete", { files }),